The effect of ordering on the strength and dislocation arrangements in the Ni₃Mn superlattice

Abstract

Abstract The dislocation configurations in thin foils of Ni3Mn have been examined by transmission electron microscopy for various intermediate states of long-range order as well as for complete long-range disorder. It has been found that, as long-range order commences, pairs of ordinary dislocations, i.e. superlattice dislocations, are formed. The spacing between these pairs decreases as the order becomes more perfect. Due to its relatively low stacking-fault energy, a high density of stacking-fault ribbons has been found to be present in the disordered alloy. Upon ordering, however, these faults are completely eliminated. This behaviour has been associated with an effective increase in the stacking-fault energy due to long-range order. In addition, measurements of the flow stress show a pronounced increase in strengthening for the intermediate states of order. This behaviour has been discussed in conjunction with the present theories of order strengthening, all of which fail in one way or another to account suitably for the observed results. Another mechanism has been proposed which is associated with the destruction of the component of short-range order co-existing with long-range order, and is found to account qualitatively for the present results.